Optical tracking apparatus

ABSTRACT

This invention consists of an optical tracking apparatus comprising a source of parallel light, such as laser, which irradiates the object to be tracked. A light scanning device, which is disposed between the source of parallel light and the object, is provided to deflect or scan the light from the source across the object to be tracked. A convex lens is provided to focus light reflected from the object. A light detector is provided which receives the reflected light and converts it into an electrical signal. The output of the light detector is fed to a phase detector which senses its phase with reference to the phase of the scanning device. Deviation of the scanning center of the scanned light from a selected position on the object to be tracked is detected, and a servo-system is provided to reduce the deviation to zero.

Sept. 4, 1973 OPTICAL TRACKING APPARATUS Inventors: Kazuo Okada,Suita-shi; Shlgeru Ando, Toyonaka, both of Japan [73] Assignee:Mitsubishi Denki Kabushiki Kaisha,

Tokyo, Japan [22] Filed: Dec. 17, 1971 21 Appl. No.: 209,061

[30] Foreign Application Priority Data Dec. 26, 1970 Japan 45/123921Dec. 26, 1970 Japan 45/123923 Oct. 13, 1971 Japan 46/80740 US. Cl250/202, 219/125 PL, 250/234 Int. Cl. G061: 11/02 Field of Search250/202, 234, 219 QA;

224,178 10/1972 U.S.S.R ..250/202 Primary Examiner-James W. LawrenceAssistant Examiner-T. N. Grigsby Attorney-Oblon, Fisher & Spivak [57]ABSTRACT This invention consists of an optical tracking apparatuscomprising a source of parallel light, such as laser, which irradiatesthe object to be tracked. A light scanning device, which is disposedbetween the source of parallel light and the object, is provided todeflect or scan the light from the source across the object to betracked. A convex lens is provided to focus light reflected from theobject. A light detector is provided which receives the reflected lightand converts it into an electrical signal. The output of the lightdetector is fed to a phase detector which senses its phase withreference to the phase of the scanning device. Deviation of the scanningcenter of the scanned light from a selected position on the object to betracked is detected, and a servo-system is provided to reduce thedeviation to zero.

4 Claims, 59 Drawing Figures I0 I I EXCITER LASER i .4 l 5 5 5, I2 PHASEPOWER l DETECTOR AMPLIEIER PAIENIEBSEP 4W 3.157. 12s

saw 01 or 12 IEXCITER FIG.

IN VENTORS KAZUO OKADA SHIGERU ANDO BLHWWMLM Pmiminsv' 3.757. 125

SHEET 02 0F 12 TIME /7/77/7 INTENSITY OF THE REFLECTED LIGH I ,/\J

FIG. 2A a I f/\ 6-\ FIG. 28 W TIME 6 I INTENSITY OF THE REFLECTED LIGHTF|G.2B' TIME A A W TIME 6 INTENSITY OF THE REFLECTED LIGHT FIG. 26'

1 TIME FIG.4

OUTPUT VOLTAGE OF THE PHASE DETECTOR POSITION 0F GAP FIG.9

OUTPUT VOLTAGE OF THE PHASE IITECTOR POSITION OF THE EDGE Fm PAIENTEDSEP4m 3.157.125

sum as or 12 FIG.5

AMPLIFIER LASER PHASE DETECITOR PATENTEU 4373 3,757. 125

sum U8UF12 mmmw ma 3.757.125.

SHEE as or 12 OUTPUT OF THE PHASE DETECTORTOPTIONAL SCALE) FIG. 17

OUTPUT VOLTAGE OF THE PHASE DETECTOR Z POSlTlON OF THE EDGE fi'm mamm 4m3.767.125

sum 100F124 AMPLIFIER FIG.I3

PHASE DETECTOR INTENSITY OF THE UGHT TRANS- SHEET 12 BF 12 0 w TIME E V2%, 24 FIG. 1 FIG. I6E E 3 TRANSMISSION RATE OF THE DENSITY WEDGE 5%W mmm 8 OI mm c; o;

0 TIME 7 5 FIG. I6C

E0 I80 360 TIME 2%; FIG. I6D' E35 24 c I g 3| 22 I T E EEO 180 560 IM oLLI OPTICAL TRACKING APPARATUS A slit plate, which has a slit orientedsubstantially parallel to the scanning direction, may be providedbetween the lens and the light detector. Alternatively, an opticaldensity wedge may be provided, having a transmission factor which variesin a direction orthogonal to the scanning direction, and on which thelight from the lens is focused.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to optical tracking apparatus, and more particularly to anapparatus which tracks an object, or the position of the clearancebetween two bodies, or a marking line, or an edge line of a body, etc.,without physical contact.

2. Description of the Prior Art In, for example, the automation ofwelding, it is required to detect the position of the butt line betweentwo plates, and a marking line, or white line drawn along a desired weldline. l-leretofore, gaps or marking lines, etc., have been detected by acontact piece. The use of contact pieces, however, has beenunsatisfactory in that the contact pieces may cause tracking errors dueto high welding temperatures. In addition, since the contact pieces maynot be brought into the vicinity of an actual welding point, they may beparticularly inaccurate in tracking curved weld lines.

In the automation of a fillet weld, for example, it is necessary todetect the edge between two plates to be welded. The most common meansfor detecting the edge is to use a contact piece which follows the edgeprofile. However, such devices have been disadvantageous in that errorsare caused by the heat of welding, and, where a weld line is curved,unless the spacing between the welding position and the detectingposition is small, the error is large. Yet the spacing may not be madesmall enough with the contact system, due to the effect of heat on thecontact piece.

SUMMARY OF THE INVENTION Accordingly, one object of this invention is toprovide a novel optical tracking apparatus which does not requirephysical contact with a workpiece.

Another object of this invention is to provide a novel trackingapparatus which is reliable and highly accurate in following weld linesof any shape.

Yet another object of this invention is the provision of a novel opticaltracking apparatus capable of detecting both linear and angularmovements of a workpiece.

Briefly, these and other objects of the invention are achieved byproviding an optical tracking apparatus comprising a source of parallellight which irradiates the body to be tracked, a light scanning devicewhich is disposed between the source of parallel light and the bodywhich deflects and scans the light from the source of parallel lightacross the object to be tracked. A lens is provided which focuses lightreflected from said body. A light detector detects focused light, and aservo-system detectsfthe phase of the output of the light detector withreference to the phase of the deflection or scanning device. Theservo-system detects the deviation of the scanning center of the scannedlight from the center of said object to be tracked, and effects acontrol so as to make said deviation zero.

BRIEF DESCRIPTION OF THE DRAWINGS A more complete appreciation of theinvention and many of the attendant advantages thereof will be readilyobtained as the same becomes better understood by reference to thefollowing detailed description when considered in connection with theaccompanying drawings wherein:

FIG. 1 is a block diagram showing one embodiment of the optical trackingapparatus according to this invention;

FIGS. 2A-2C and 2A'2C are three part graphical diagrams for explainingthe principle of the detection of a gap using the apparatus of FIG. 1;

FIG. 3 is a schematic diagram showing one technique for the detection ofa marking line using the apparatus of FIG. I;

FIG. 4 is a graphical diagram showing an output char acteristic of theapparatus of FIG. 1;

FIG. 5 is a perspective and block diagram showing another embodiment ofthe optical tracking apparatus according to this invention;

FIGS. 6a, 6b, and 6b are diagrams explaining the principle of thedetection of an edge using the apparatus of FIG. 5;

FIGS. 7a-7c and 7a'7c' are diagrams explaining the principle of thedetection of an edge using the apparatus of FIG. 5;

FIGS. 8A-8D and 8B'-8D' are diagrams explaining the principle of thedetection of an edge using the apparatus of FIG. 5;

FIG. 9 is a graphical diagram showing an output characteristic of theapparatus of FIG. 5;

FIGS. l0A-10D and 10B'l0D are diagrams explaining the principle of thedetection of a displacement using the apparatus of FIG. 5;

FIGS. llA-llC and llA'-l 1C are diagrams explaining the principle of thedetection of an angle using the apparatus of FIG. 5;

FIG. 12 is a graphical diagram showing an output characteristic of theangle detection apparatus of FIG.

FIG. 13 is a perspective and block diagram showing still anotherembodiment of the optical tracking apparatus according to thisinvention;

FIGS. 14a, 14b, 14a and 14b are diagrams explaining the principle of thedetection of an edge using the apparatus of FIG. 13;

FIGS. Isa-15c and l5a'-l5c' are diagrams explaining the principle of thedetection of an edge using the apparatus of FIG. 13; y

FIGS. l6A-16D and 16B-l6D are diagrams explaining the principle of thedetection of an edge using the apparatus of FIG. 13;

FIG. 17 is a graphical diagram showing an output characteristic of theapparatus of FIG. 13.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawingswherein like reference numerals designate identical or correspondingparts throughout the several views, and more particularly to FIG. 1thereof, one embodiment of the present invention is illustrated. In thefigure, numeral 1 designates a source of parallel light, such as laser 2a light scanning device which deflects and scans the parallel light inthe direction of y-axis at a recurrence frequency 1}, (the deflectionand scanning being hereinafter simply termed the scanning"), 3 anexciter device or scanning signal generator which actuates the lightscanning device, 4 and 5 reflectors which direct the scanned light(scanning light) to a workpiece or specimen 6 having a gap 6g. Thenumeral '7 denotes a lens which condenses light reflected from thespecimen, 8 a light detector, such as a photocell, which converts thecondensed light into an electric signal, 9 a moving stand which moves inthe direction of the y-axis, 10 a phase detector device which detectsthe phase of the output signal of the light detector 8 with reference toa signal from the exiter device 3, ill a power amplifier, I2 aservomotor which drives the moving stand 9 according to the output ofthe power amplifier. The numeral 13 represents the light beams.

The principle of operation of the present invention will now bedescribed. Although the parallel-light source 1 may also be acombination of a point source of light and a lens, a laser is typicallyemployed in order to enhance the precision of the apparatus. The lightemanating from the parallel-light source I is scanned in parallel withthe y-axis in FIG. I at the recurrence frequency f by means of the lightscanning device 2. Used as the light scanning device is, for example, ameans which utilizes a well-known electrooptical effect. The scanninglight is projected over the gap portion of the specimen 6 through thereflectors 4 and 5. The width of the scan is selected so as to beslightly larger than the gap in the workpiece or specimen. In FIG. I,there is depicted a situation where the center of the scanning lightcoincides with the center of the gap. The scanning light incident on thespecimen is reflected when the light beam impinges on the gap. Thereflected light is condensed and focused on the light detector 8 by thelens 7, and is converted into an electric signal representative of theintensity of the light. This electric signal is detected by the phasedetector It), with a signal from the exciter device 3 acting as areference signal.

The functions thus far described will be explained in detail withreference to FIGS. 2A-2C and 2A-2C'. FIGS. 2A, 2B and 2C illustrate theprogress-versustime of the light beams 13 of the scanned light projectedover the gap portion of the specimen. FIGS. 2A, 2B and 2C illustrate theprogress-versus-time of the intensity of the reflected light from thespecimen 6. FIGS. 2A and 2A show a situation where the scanning centerand the center of the gap deviate, FIGS. 2B and 2B show a situationwhere they are coincident, and FIGS. 2C and 2C show a case where theydeviate in the opposite sense from that of FIGS. 2A and 2A.

In the respective figures, the time axes are depicted graduated in termsof phase. In case of FIG. 2A, the intensity of the reflected lightvaries by one period during one period of scanning. It is to beunderstood that, when the scanning center and the gap center are broughtinto coincidence, as in FIG. 2B, the intensity of the reflected lightvaries two periods or cycles during one scanning period. In FIG. 2C,there appears a reflected-light intensity which is the same in periodas, but differs in phase (by 1r or 180") from, the case of FIG. 2A. Morespecifically, the intensity of the reflected light in the case of FIG.2B has a varying component of frequency 2f while a varying component offrequency f appears in the cases of FIGS. 2A and 28, with the resultthat the phase difi'ers by 1r between the cases of FIGS. 2A and 2C asdescribed above. When the relative position between the scanning centerand the gap center is intermediate between the cases of FIGS. 2A and 2Bor between those of FIGS. 23 and 2C, variations in the intensity of thereflected light have components of both frequencies f, and 2}}.

It is apparent from the above explanation that, if the output signal ofthe light detector is phase-detected, with the signal of the exciterdevice as a reference Signal, an output signal of a characteristic as inFIG. 4 is obtained. This signal is amplified by the power amplitier II,the amplified signal is applied to the servomotor l2, and the movingstand 9 is driven with the polarities of the servomotor suitablyselected. Then, the apparatus may be controlled so that the scanningcenter and the gap center may be brought into coincidence. When thespecimen or the moving stand is moved in the direction of z-axis, themoving stand 9 normally detects and tracks the profile of the gap.Accordingly, if by way of example, the present invention is applied to awelding machine, and a welding torch is placed on the moving stand, theapparatus automatically detects curves in the gap or weld lines, andcarries out welding.

FIG. 3 shows an embodiment in which, instead of the gap in FIG. I, amarking line or white line is detected. When the light is passed acrossthe marking or white line in the scanning step, the light is reflectedat the position of the line and is projected into the lens 7. When thescanning center corresponds to the marking line, a frequency component2f results in the intensity of the reflected light. When the scanningcenter is varied in the direction of y-axis from the marking line, acomponentof frequency f,, results. The phenomenon may be more clearlyunderstood by considering the previously described gap as the markingline of FIG. 3. Accordingly, the moving stand 9 can be servocontrolledso as to make the scanning center correspond to the marking line by theout-put of the system. In accordance with the embodiments of thisinvention, the gap, the marking line, or the white line, etc., can bescanned to control welding and other automatic apparatuses.

The system of the present invention has the advantage of no contact withthe workpiece, as well as the advantage that the effects of otherspurious light can be minimized by using a laser. In addition, a highratio of signal to noise is provided whereby a high accuracy is obtainedin the detector. Incidentally, although in the embodiments of thisinvention, there are illustrated by way of example the gap,'the markingline and the white line, the invention is not intended to be limited inany manner.

FIG. 5 shows another embodiment according to the present invention. Inthe figure, the numeral 21 indicates a source of parallel light, such aslaser, 22 a light scanning device which deflects and scans the light inthe direction of the x-axis (the deflection and scanning beinghereinbelow simply termed the scanning). The numeral 23 denotes a lightdetector or transducer which converts into an electric signal the lightfocused by a lens 25 and received through a slit plate 24 having a slit24a. The lens 25 focuses on the slit plate 24 the light reflected from aspecimen body. The numeral 26 denotes a moving stand, which is driven inthe direction of the x-axis in the drawing by means of a driving motor29. The parallel-light source 2H, the light scanning device 22, thelight detector 23, the slit plate 24, and the lens 25 are mounted on themoving stand 26. Numeral 27 represents a phase detector which detectsthe phase of the output signal from the light detector 23 with referenceto a signal from the light scanning device 22. The numeral 28 designatesa power amplifier which amplifies an output of the phase detector 27,and 29 represents a servomotor which drives the moving stand 26 throughthe output of the power amplifier 28. The numeral 3@ designates the bodyto be measured which consists of two faces K and L which have a commonedge The numerals 31 and 31' denote the loci of the light beams.

The operating principles of this embodiment will now be described.Although the parallel-light source 21 may also be a combination of apoint source of light and a lens, a gas laser is typically employed inorder to enhance the precision of the apparatus. The light emanatingfrom the parallel-light source 21 is scanned in parallel with the x-axisillustrated in FIG. 5 at a recurrence frequency of j}, by means of thelight scanning device 22. The light scanning device 22 may be a meanswhich utilizes a well-known electooptical effect, for example. Forsimplicity, the scanning device 22 may consist of a plane mirror stuckto one end of a tuning fork of a tuning fork oscillator. The parallellight is then caused to impinge upon the plane mirror, whereby a scannedand reflected parallel light may be obtained. The scanning angle AB istypically 2.5 X 10 radians or so, and, for example, where the distancebetween the light scanning device 22 and the body to be measured 34) is200mm, the width of deflection of the parallel ligliis 5 mm on the bodyto be measured. An optical axis be of the parallel-light source 21 andan optical axis E5 of the lens 25 are contained in the yz-plane asshown, and both the optical axes are so set as to define an angle of orwithin the plane. Typically, the angle a is approximately 2 X 10radians.

Loci of the scanned parallel light on the surfaces of the body to bemeasured 30, are shown in FIGS. 6a, 6b, 6a an d 6b. FIG. 6a illustratesa case where the optical axes b and a? are coincident. Here, the locusof light as observed in the direction from a to 0 along the y-axisbecomes a straight line, parallel to the x-axis as also shown in FIG.6a. In contrast, in the case where the observationaldirection E definesthe angle a with the optical axis 120, as shown in FIG. 6b, the locus oflight as viewed in the direction from a to 0 becomes a polygonal line,as shown in FIG. 6b.

The body to be measured 30 consists of both the planes K and L. The edge71F thereof passes through the point 0 and is parallel to the z-axis,and an angle which the planes K and L define with the x-axis,respectively, is typically 45". The locus of the scanned light in thiscase appears, as illustrated in FIGS. 7a-7c and 74- '-7c'. In FIG. 7b aituation is illustrated where the scanning center axis ha of the scannedparallel light and the edge of the body to be measured 30 are broughtinto coincidence. If the position of the body to be mea sured is changedas in FIGS. 7a and 7c, the corresponding loci of the scanned lightbecomes as shown in FIGS. 7a and 70'.

Next, the light reflected from the planes of the body to be measured 30is condensed by the lens 25, and is focused on the slit plate 24oriented parallel with the xz-plane. The slit 24a is disposed inparallel with the xaxis in FIG. 5, and its width is approximately equalto the width of the light locus 31 on the slit plate 24. The position ofthe slit plate 24 in the direction of z-axis is adjusted as shown inFIG. 5, so that in the case where the edge of the body to be measured iscoincident with the optical axis 5 (in the case of FIG. 7b), the bentportion may coincide with the slit.

The light having passed through the slit 24 impinges on the lightdetector 23, and the intensity thereof is converted into an electricsignal to be fed into the phase detector 27.

The wave form of the drive voltage of the light scanning device 22 isused as the reference signal of the phase detector 27. A positive ornegative DC signal appears on the output side of the phase detector 27,depending upon whether the phase difference between the reference signaland the signal from the light detector 23 is 0 or The functions of theapparatus as thus far described, will be explained in more detail withreference to FIGS. 8A-8D and 8B'8D. The loci of light beams focused onthe slit plate 24 are shown in FIGS. 8B, 8C and 8D, while thecorresponding progress-versus time of the loci of light beams focused onthe slit and the progressversus-time of the intensities ofslit-permeating light are illustrated in FIGS. 83', 8C and 2D,respectively. In the respective figures, the time axes are depicted asgraduated in terms of the phase. FIG. 8B is directed to the case vherethe edge H5 is coincident with the optical axis b0, FIG. SC to the casewhere the edge W (and accordingly, the body to be measured) is displacedin the negative direction of the x-axis, and FIG. 8D to the case whereit is displaced in the positive direction of the same axis. As isapparent from FIG. 813, when the edge 71? is coincident with the opticalaxis 1?, the intensity of the slit-permeating light varies by twoperiods while the parallel light effects one period of scanning. Thatis, the intensity of the slit-permeating light has a frequency componentof 2f}. When the body to be measured is displaced in the negativedirection of the x-axis as shown in FIG. BC, the slit-permeating lighthas a frequency component of f}. When the body to be measured isdisplaced in the positive direction of the x-axis, there is obtained apermeating light which, as shown in FIG. 8D, has a frequency componentof varying in phase by 180 from the case of FIG. 8C. When thedisplacement is intermediate between the cases of FIGS. 88 and 8C orbetween those of FIGS. 88 and 8D, the frequency components of j], and2f, coexist, and the intensity of the component of f], is proportionalto the displacement for small displacements. Accordingly, when theoutput of the light detector 23 is phase-detected, an output of acharacteristic as shown in FIG. 9 is obtained.

The signal is amplified by the power amplifiers 28, the amplified signalis applied to the servomotor 29, and the fged stand 26 is thus driven,whereby the optical axis b0 may be brought into coincidence with theedge 757i.

From the foregoing description, it is clear that the functions of thewhole system are as set forth below.

An indication of the deviation between the scanning center of thescanning-light locus and the slit is obtained from the phase detector27. This locus corresponds to the displacement of the edge of the bodyto be measured 30 in the x direction and appears on the slit plate 24.Upon this indication, the servomotor 29 is rotated to move the movingstand 26. The position of the scanning center is thereby moved to theposition of the slit. Unless the edge rfi is moved again, the abovestate is maintained. If the edge "W is displaced,

the moving stand 26 effects tracking so as to make the displacementzero.

Accordingly, if, for example, the body to be measured is a body to bewelded and 1% is a weld line, the feed stand tracks the weld line. Ifmeans is also provided to move the feed stand in the direction of thezaxis, and a welding torch, for example, is placed on the feed stand, itbecomes possible to automatically track the weld line and to carry outthe welding even when the weld line is curved.

It will now be described that, as another function of the presentinvention, a displacement in the direction of the y-axis, or a distancein the direction of the y-axis, or a distance from the lens 25, may bedetected. In FIG. WA, there is shown a case where, in the constructionaldiagram of FIG. 5, it is assumed that the body to be measured 30 hasonly the surface K, and that the normal to the surface K be contained inthe xy-plane and defines an angle a with the optical axis E. The body isrepresented by the numeral 30. When as in FIG. MIC, the surface of thebody to be measured 36) is locatg at the intersection point between theoptical axis b0 and the optical axis 5 0 the locus of light on the slitplate 24 is such that the scanning center of the scanning light islocated on the slit as shown in FIG. C. Therefore, the scanning lightpasses through the slit twice during one period of scanning, and asignal having a frequency component of 2f,, appears. On the other hand,when the body to be measured 30 is displaced towards the lens (FIG. MB)or in the direction opposite thereto( FIG. 101)) on the y-axis, thescanning light is passed through the slit only once during one period ofscanning, and a signal of a frequency component f appears. Thus, thephase differs by 180 depending upon on which side of o the body to bemeasured 30 is located. Therefore, an output characteristic as in FIG. 9may also be obtained from the phase detector. The moving stand 26 may beservo-driven in the direc tion of the y-axis by means of the outputsignal, whereby the intersection point 0 may be always positioned on theplane of the body to be measured 30. Consequently, the displacement ofthe surface of the body to be measured in the y-axial direction may beread by reading the movement of the moving stand 26.

Referring to FIGS. llA-IIC and IIAIIC' a technique will be described formeasuring the angle 0, as still another function of the presentinvention. It is now assumed that, as in the figure, the surface of thebody to be measured 30' is slightly displaced onto the lens side on they-axis. If the angle is 0 as shown in FIG. 11A, the scanning light ispassed through the slit on the slit plate 24 once during one period,and, hence, a signal having a frequency component f, is generated in thelight detector 23. The intensity of the signal becomes lower as theangle 0 approaches zero, and the signal is zero at 0=0 as in FIG. IIB.When the situation is as shown in FIG. 11C, the frequency component 1;,appears again. However, the phase thereof differs by 180 from that ofthe situation shown in FIG. 1111A. Accordingly, the output of the phasedetector typically becomes as shown in FIG. 12.

The predicted results were obtained in an experiment in which a piece ofplain white paper was used for the surface to be measured. In spite ofthe fact that the dependency upon the angle 0 of the intensity of lightincident on the lens 25 was not corrected, outputs substantiallyproportional to the angle were obtained within the range of 20 0 20.Thus, although non-contact measurements or control of an angle have beenconsidered as being difiicult in comparison with the measure ment of adisplacement, they are made possible using the present inventionaccording to the above method.

Since the apparatus shown in FIG. 5 is constructed and operated asexplained above, it is efiective in tracking the edge of the specimenwithout any physical contact with the specimen.

FIG. 13 shows still another embodiment according to the presentinvention. The embodiment of the invention illustrated in FIG. 13 isidentical with that illustrated in FIG. 5, except that the slit plate ofthe FIG. 5 embodiment is replaced with an optical density wedge 34 inthe FIG. 13 embodiment. The optical density wedge, or density wedge 34has a transmission factor gradient which increases in the positivedirection of the z-axis shown in FIG. 13.

Loci of light which the scanned parallel light forms on the surfaces ofthe body to be measured 30, are shown in FIGS. Ma, 14b, Ma and Mb whichare similar to FIGS. 6a, 6b, 6a and g). FIG. Ma illustrates a case wherethe optical axes b0 and a 0 are coincident. Here, the locus of light, asobserved in the direction from a to 0 along the y-axis, becomes astraight line parallel to the x-axis as shown in FIG. 14a. In contrast,in a case where the observationahlirection (E defines the angle a withthe optical axis be as shown in FIG. 14b, the locus of light as viewedin the direction from a to 0 becomes a polygonal line as shown in FIG.14b.

The body to be measured 3t) consists of both the surfaces K and L, theedge 51? thereof passes through the point 0 and is parallel to thez-axis. An angle which the surfaces K and L define with the z-axis,respectively, is typically 45. The locus thereof appears, as illustratedin FIGS. 15a 15c and 15a to 15 which are similar to FIGS. 7a 7c and 7aIn the case of FIG. 15b, a situation is illustrated where the scanningcenter axis be of the scanned parallel light and the edge rwz of thebody to be measured 30 are brought into coincidence. If the position ofthe body to be measured is changed as in FIGS. 15a and 15c, thecorresponding loci of the scanned light become as shown in FIGS. Isa and15b.

Next, the light reflected from the faces of the body to be measured 30is condensed by the lens 25, and is focused on the density wedge 34oriented parallel with the xz-plane. Since the density wedge 34 hasvariations in its transmission factor in the z a.xial direction, theintensity of light transmitted through the density wedge 34 variesdepending upon the incident position of the light in the direction ofthe z axis.

The light having passed through the density wedge 34 impinges on thelight detector 23, and the intensity thereof is converted into anelectric signal to be fed into the phase detector 27.

Again, the wave form of the driving voltage of the light scanning device22 is used as a reference signal for the phase detector 27. A positiveor negative DC signal appears on the output side, depending upon whetherthe phase difference between the reference signal and the signal fromthe light detector 23 is 0 or The functions of the apparatus, as thusfar described, will be explained in more detail with reference to FIGS.16A 16D and 168' to MD which are similar to FIGS. 8A 8D and 8B 8D. FIG.MA illustrates the progress-versus-tirne of the scanning wave form ofthe scanned light. The loci of light beams focused on the density wedge34 are shown in FIGS. 16B, 16C and 16D. The progress-versus-time of theloci of light beams focused on the density wedge and theprogressversus-time of the intensities of the light transmitted throughthe density wedge are shown in FIGS. 16B, 16C and 16D, respectively. Thetransmission factor characteristic of the density wedge is shown in FIG.16E. In the respective figures, the time axes are depicted graduated interms of the phase. FIG. 16B is directed to the case where the edge H7:(that is, the body to be measured) is displaced in the negativedirection of the x-axis, and FIG. 16D to the case where it is displacedin the positive direction of the same axis. As apparent from FIG. 1613,when the edge m nis coincident with the optical axis 1;, the intensityof the light transmitted through the density wedge varies by two periodsduring one period of scanning by the parallel light. Thus, as explainedabove, the intensity of the light transmitted through the density wedgehas a frequency component of 2f}. When the body to be measured isdisplaced in the negative direction of the x-axis as in FIG. 16C, thelight transmitted through the density wedge has a frequency component off When the body to be measured is displaced in the positive direction ofthe x-axis, there is obtained a transmitted light which, as shown inFIG. 16D, has a frequency component of f, varying in phase by 180 fromthe case of FIG. 16C. When the displacement is intermediate between thecases of FIGS. 16B and 16C or between those of FIGS. 16B and 16D, thefrequency components of f, and 2f}, coexist, and the intensity of thecomponent of f, is proportional to the displacement, for smalldisplacements. Accordingly, when the output of the light detector 23 isphase-detected, an output characteristic as shown in FIG. 17 isobtained.

The signal is again amplified by the power amplifier 28, the amplifiedsignal is applied to the servomotor 29, and the fe ed stand 26 is thusdriven, whereby the optical axis be may be brought into coincidence withthe edge W.

When the distance from the edge iii: to the lens 25 changes, the lightlocus 31' on the density wedge 34 moves in the z-axial direction. If,however, the width of the density wedge in the z-axial direction is madesufficiently large so that the light locus 31 does not overlap thedensity wedge 34, then the change in the detection sensitivity due tothe fluctuation in the distance is small. This is an advantageousfeature in a case where the distance fluctuations are withinapproximately fimm as in the detection of weld lines.

From the foregoing description, the functions of the FIG. 13 system as awhole are as set forth below.

An indication corresponding to the displacement of the edge of the bodyto be measured 30 in the x direction, is obtained from the phasedetector 27. The servomotor 29 is rotated in response to the indicationto thereby move the stand 26 and in turn to move the position of thescanning center to that of the edge. Unless the edge iiii is movedagain, the above state is maintained. If the edge 7n? is displaced, themoving stand 26 effects tracking so as to render the displacement zero.

Accordingly, if, by way of example, the body to be measured is a body tobe welded and the edge HE is a weld line, the feed stand tracks the weldline. If means are also provided to move the feed stand in the z-axialdirection, and a welding torch, for example, is placed on the feedstand, the weld line is automatically tracked and the welding carriedout even when the weld line is curved.

Again, the apparatus of FIG. 13 is capable of tracking the edge of thespecimen with absolutely no physical contact with the specimen.

Obviously, numerous additional modifications and variations of thepresent invention are possible in light of the above teachings. It istherefore to be understoood that within the scope of the appended claimsthe invention may be practiced otherwise than as specifically describedherein. 1

What is claimed as new and desired to be secured b letters patent of theUnited States is:

1. An apparatus for optically tracking a body comprising:

a source of parallel light for irradiating a portion of said body,

light scanning means positioned between said source and said body forscanning said parallel light across said portion of said body,

lens means for condensing light reflected from said portion of saidbody,

light detector means for sensing said reflected light and for producingan output signal in response thereto; and,

control means responsive to said output signal for said light detectormeans for detecting deviations of said scanning light from said portionof said body, said control means operative to minimize said deviations,

slit plate means having a slit therein oriented substantially parallelto the scanning direction of said light scanning means,

said slit plate means positioned between said lens means and said lightdetector means.

2. An apparatus as in claim 1 for optically tracking a body furthercomprising:

an optical density wedge positioned between said lens means and saidlight detector means,

said optical density wedge having a transmission factor which varies ina direction orthogonal to the scanning direction of said light scanningmeans.

3. An apparatus as in claim 1 for optically tracking a body wherein:

said portion of said body irradiated by said parallel light source is anedge portion.

4. An apparatus as in claim 3 for optically tracking a body wherein:

said light scanning means deflects light from said source substantiallyorthogonally with respect to said edge portion.

1. An apparatus for optically tracking a body comprising: a source ofparallel light for irradiating a portion of said body, light scanningmeans positioned between said source and said body for scanning saidparallel light across said portion of said body, lens means forcondensing light reflected from said portion of said body, lightdetector means for sensing said reflected light and for producing anoutput signal in response thereto; and, control means responsive to saidoutput signal for said light detector means for detecting deviations ofsaid scanning light from said portion of said body, said control meansoperative to minimize said deviations, slit plate means having a slittherein oriented substantially parallel to the scanning direction ofsaid light scanning means, said slit plate means positioned between saidlens means and said light detector meAns.
 2. An apparatus as in claim 1for optically tracking a body further comprising: an optical densitywedge positioned between said lens means and said light detector means,said optical density wedge having a transmission factor which varies ina direction orthogonal to the scanning direction of said light scanningmeans.
 3. An apparatus as in claim 1 for optically tracking a bodywherein: said portion of said body irradiated by said parallel lightsource is an edge portion.
 4. An apparatus as in claim 3 for opticallytracking a body wherein: said light scanning means deflects light fromsaid source substantially orthogonally with respect to said edgeportion.